ARM64: Move xSELF from x18 to x19.

This patch moves xSELF to callee saved x19 and removes support for
ETR (external thread register), previously used across native calls.

Change-Id: Icee07fbb9292425947f7de33d10a0ddf98c7899b
Signed-off-by: Serban Constantinescu <serban.constantinescu@linaro.org>
diff --git a/runtime/arch/stub_test.cc b/runtime/arch/stub_test.cc
index a7d24b8..23b7cfa 100644
--- a/runtime/arch/stub_test.cc
+++ b/runtime/arch/stub_test.cc
@@ -173,7 +173,7 @@
         // Load call params into the right registers.
         "ldp x0, x1, [sp]\n\t"
         "ldp x2, x3, [sp, #16]\n\t"
-        "ldr x18, [sp, #32]\n\t"
+        "ldr x19, [sp, #32]\n\t"
         "add sp, sp, #48\n\t"
         ".cfi_adjust_cfa_offset -48\n\t"
 
@@ -526,7 +526,7 @@
         // Load call params into the right registers.
         "ldp x0, x1, [sp]\n\t"
         "ldp x2, x3, [sp, #16]\n\t"
-        "ldp x18, x17, [sp, #32]\n\t"
+        "ldp x19, x17, [sp, #32]\n\t"
         "add sp, sp, #48\n\t"
         ".cfi_adjust_cfa_offset -48\n\t"